Around 0500hours on 20 September 1943, the German submarine U-270 fired a Gnat torpedo at HMS Lagan (Lt.Cdr. Albert Ayre, RNR (retired)) near convoy ON-202. The torpedo struck her in the stern. The position of the attack was 57º09'N, 27º28'W. The U-boat was then chased away by depth charges from the Canadian destroyer HMCS Gatineau and the badly damaged frigate was towed to the Mersey where she arrived on 24 September. Damage was severe and she was declared a Constructive Total Loss (CTL). Damage sustained from torpedo was 30 feet of stern from frame 115 aft missing. 30 feet of after deck from frame 100 badly damaged. Forward superstructure damaged by debris including tinned food and depth charges raining down. One officer and 28 ratings were killed or missing.

Notable events involving Lagan include:

12 May 1943German U-boat U-89 was sunk in the Northern Atlantic, in position 46°30'N, 25°40'W by a Swordfish aircraft (Sqdn. 811) from the British escort carrier HMS Biter (Capt. E.M.C. Abel Smith, RN), the British destroyer HMS Broadway (Lt.Cdr. E.H. Chevasse, RN) and the British frigate HMS Lagan (Lt.Cdr. A. Ayre, RNR).

13 May 1943German U-boat U-753 was sunk in the North Atlantic, in position 48°37'N, 22°39'W, by depth charges from the Canadian corvette HMCS Drumheller (Lt. L.P. Denny, RCNR), the British frigate HMS Lagan (Lt.Cdr. A. Ayre, RNR) and depth charges from a Canadian Sunderland aircraft (RCAF Sqdn. 423/G).